High frequency wattmeter



Jan. 26'; 1943. E. MITTELMANN HIGH FREQUENCY WATTMETER Original FiledApril 22, 1938 lllllllllllllllllllll mo 1s .10 as Reiaued Jan. 26, 1943UNITED STATES A PATENT OFFICE I mun rumor- 311:: wamm'rm EugenMittelmann, Chime, Ill. Original No. 2,240,955, dated May 6, 1941,Serial .No.203,661,.April 22, 1938.

issue Mayli, 1942, Serial No. 441,921;

' tria April 26, 1937 Application for re- In Aus- 15 Claims. (Cl.I'll-95) This invention relates to a method of and an apparatus formeasuring the power which is absorbed by. the load only in the outputcircuit of a high frequency generator.

While ,my invention is applicable to the measurement or determination ofthe power absorbed by any load in the output circuit of a high frequencygenerator, the invention has special applicatlon to the determination ofpower absorbed by a patient from the output circuit of a short wave orhigh frequency therapy machine.

v An object of my invention is to provide a method of and apparatus forthe measurement or determination of the load component oi the totalpower output of an oscillatory circuit, which component may be referredto as that portion of the total power output which is actualiy used toenergize the load, as distinguished from those portions of the totalpower output which constitute power losses in, or radiation losses zrom,the circuit or apparatus. In the appiicatiou of short wave therapy, onlya fraction oi? the totai high frequency power output of the apparatus isabsorbed by the patient undergoing treatment,

andthe remainder is lost through radiation,

leakage, etc. Known measuring devices with which I am familiar indicatemerely the total power output oi the output circuit, which is includedthe heating device or electrodes.

An object of my invention is, accordingly, to measure the power absorbedby the patient independently of the total power output of the circuit.

Other objects and advantages oi my invention will be readily understoodby reierenceto the accompanying drawing, wherein is illustrateddiagrammatically the output circuit of a short wave or high frequencygenerator embodying my invention.

Referring to the accompanying drawing- Figure l is a diagrammatic viewof the secondary or outputv circuit of a high frequency zenerator,showing the instrumentalities which -I utilize in carrying out myinvention; and

Figure 2 is a similar view to Figure 1 in which a modified form ofinstrumentality is utilized.

In both of the figures of the drawing, A represents a source of highfrequency oscillations coupled to a secondary or output circuit B bymeans of a transformer C, all in a manner well known in the art. Theapparatus forming the The resonance output circuit B is shown asincluding conductors l and 2 connecting the secondary of the transformerC withsuitable electrodes 5 which are spaced apart, and between whichthe object 6 is positioned. The object 5 represents a load,absorbing'power from the output circuit B. It is the power absorption ofthis load or object which is to be measured.

Ifthe high frequency is to he applied as treatment to a patient, then itis to be understood that the object G-is intended to represent thepatient's body or some portion thereof positioned between theelectrodes, as is well known and generally understood in this art. Theoutput circuit in apparatus of this character generally has a variablecondenser 4 bridged across the circuit for the purpose of tuning thecircuit to resonance.

The efiect of placing a patients bodlfior some portion thereof, betweenthe electrodes in course, that other power absorbing elements alreadyexist in the circuit and are inherent thereto as characteristicsthereof.

In accordance with my invention, I utilize a measuring instrument whichoperates on the principle of the square law, that is, in which thedeflection oi the pointer is proportional to the square oi the'voltageor current. any standard or well mown device of this type may mutilized. That which is shown in Figure l is of the hot wirethermocouple and galvanometer type, while that shown in Figure 2 is ofthe dynamometer type having a moving coil and an exciting coil andutilized in connection with an electronic tube operating in the squareportion of its characteristic.

In Figure l the thermocouple 9 is, bridged across the output circuitconductors I and 2 by means of conductors 'l-I, which include thecondensers H.

The thermocouple 8 is connected to the terminals of the galvanometer 12by means of conductors lt-Jiin the usual manner in instruments of thischaracter. The galvanometer 12 has a pointer II which moves over acalibrated dial lie, the deflection of the pointer being proportional tothe square of the voltage across the condenser l, or the load inparallel therewith. This dial is calibrated in equal divisions, and, inthe present instance, I have shown the dial as calibrated in divisionsfrom 0 to 100.

In accordance with my invention, I provide a shunt circuit around thegalvancmeter, which shunt circuit includes a variable resistance ll forvarying the sensitivity'of the galvanometer. This shunt circuit includesthe conductors I! which are, connected to the terminals of thegalvanometer. One of these conductors is connected to a terminal of theresistance I4 and the other is connected to a contact member or wiper l8movable over the resistance ll. Alongside the path of the wiper I8 is ascale or dial II which is preferably calibrated in divisions from to100, corresponding to the dial of the galvanometer. The wiper carriesapointer l! which cooperates with the dial II to indicate the resistancevalues in. terms of the galvanometer readings.

Since it is desired that the shunt resistance ll vary the sensitivity ofthe galvanometer in certain mathematical ratios, as will hereinafter bexplained, it is necessary to first calibrate the dial ll of theresistance in accordance with those ratios rather than in terms of thetrue values of the resistance l4.

It indicates the same value as'that indicated by the galvanometer II,the sensitivity of the galvanometer will be of the value required tocause the galvanometer to indicate the true value of the power absorbedby the body or load I. It will be seen from latter discussions that theresistance is conveniently constructed in such amanner that .the ohmagegradient thereof will permit use of a dial l'l having linear divisions.

The principle upon which the measurement of the power absorbed by theobject is accomplished by the above describedapparatus may be explainedas follows:

The secondary resonance circuit B (of Figure 1) under no load and loadcondltions'may be denoted by two equivalent circuits. respectively,

as follows: (i) a source of high frequency voltage in series with asimple coil and a variable condenser and having a single resistance m inparallel with-the condenser; and (2) the same circuit with a secondresistance, R1 in parallel with'the resistance Rm, The Be may be definedas that resistance in parallel with the condenser which causes a powerloss equal to the no load losses of the circuit, and the resistance R1may bedeflned as that resistance inparaliel with the condenser whichabsorbs an amount of some the secondary-circuit, i. e., across thevariable condenser, under no load, is es, and the relation between onand R0 is:

ec=k.Ro (1) where k is a factor of proportionality dependent upon theresonance frequency, the capacitance or inductance, and the value of thehigh frequency source of voltage, and represents the current through theequivalent resistance Ru. From this equation it will be evident that theresonance voltage of the circuit at no load is proportional to theequivalent parallel circuit-loss resistance R0 of the circuit.

When a load orobject is introduced into the circuit between theelectrodes 5-4, additional reactive and resistive elements are coupledinto the circuit. The circuit is once more tuned to changed from sotoanew value e1.

, ance introduced into the circuit by the addition This resistance is so0811- of the load or the introduction of the body 6 may,

, brated, asfwili hereinafter appear, that when the wiper It is moved toa point such that the pointer resonance to compensate for the change inreactance of the circuit effected by the addition of the load, or theinsertion of the object. The

resonance voltage of the circuit is thereby The resistas previouslynoted, be represented by the equivalent parallel load resistance R1; Thevoltage and frequency of the source being constant, and the capacitanceof the circuit having been kept constant by the adjustment of thecondenser to tune I the circuit to the same resonance frequency, 1. e.,

' the frequency of the source or generator, the resoequation: 7

R+Ri From theabove two-equations, it will be seen that i RFRTH V powerequal to the power absorbed by'the load or object. The variablecondenser of this equivalent circuit is, ofcourse, equivalent in allre-- spects to the capacity represented by the electrodes 5-5 (inFigure 1) shunted by the variable condenser], and other contributorycapacitive factors inherent in the circuit. The inductance representsall inductive reactances present as.

part of the characteristics thereof.

The first equivalent circuit represents the I secondary circuit B withno load. or the object 6 not in position between the electrodes 'l-l.

Under these no load conditions thereare certain losses in and from thecircuit B, i. e.. leakage losses, radiation losses, 'etc., all of whichabsorb power from the" circuit.

power absorbed at no load is a function of the equivalent parallelresistance Bo. Rs is. of course, a constant for any given arrangementand spacins of the electrodes. The voltage in Since the circuit is atresonance and all reactive factors or reactancesare balanced, the totalnance voltage c1 of the circuitis proportional to the combinedresistance of the equivalent parallel resistance R0 and R1, according tothe following certain limits, which are rarely exceeded in practice,will not, at the high frequency used, alter or have any effect upon theequivalent no load resistance Rn.

By accurate control of the voltage ofthe source. of high frequency,either manually, or by voltage stabilizer, or by any other method, theno load resonance voltage may be maintained constant for any given noload circuit conditions. Thus, with en constant and R0 constant,Equation 3 becomes: I

l ti flei) or R1 is purely a function of or (4)- Keeping this inmind',and recalling that the power absorbed by R1 is thatwhich is to bedetermined, it can be seen that if we can-measure the resonance voltageof the circuit under load;

and if we know the equivalent parallel resistance added totl'ie circuitby the introduction Of the load. we can determine the power absorbed bythe load by the well known power equation:

1 P v whereE isin volts, Risinohms. and rm in watts.

Since a: is a direct function of R1 when theno load resonance voltageand no load circuit conditions are maintained constant, then under 1.those conditions the equivalent parallel resistances of the differentloads will be-determined by the values of 81. V

For given electrodes, the spacing within directly or in absolute voltagevalues.

conditions and the ratio is a constant for any given load, theequivalent parallel load resistance determine the value of theequivalent parallel load resistance R1 for any given load introducedinto the circuit.

Every change in the amount of power supplied to the secondary circuit Bso changes both the no load and the load voltages, that the conditionsof Equation 3 are always met. Hence, if we adlust resistance 14 to avalue which will render the sensitivity inversely proportional to R1determined by Equation 3, the galvanometer will indicate the powerabsorbed by the load in accordance with Equation 5. The resistance llhaving been, adjusted to this corresponding value, the galvanometer, itcalibrated in power units, will indicate the power absorbed by the sameload for any value of power supplied by the high frequency source-thismeasurement oi the power'absorption being, of course, limited only bythe range of equivalent parallel load resistance'Ri for eachload neednot be actually measured or known, and that the no load resonancevoltage co and the resonance load voltage e1 need not be measured 7 Allthat is necessary is to determine by'measurement, in a manner to bedescribed hereinafter, the ratios or quantities'aflecting the setting ofthe resistance H for any given load. The import of this is that themeasurement of the power absorbed by the load or body 6 becomes a simpleexpedient. By.

merely determining,'trom the change in the resonance voltage caused bythe addition 01 a given load to the circuit, the ratio corresponding toR1, we can, it we know any resonance voltage E applied to the given loadunder'the same electrode or circuit conditions, determine the powerabsorbed by that given load for any amount power input. This can be seenfrom a simple use of the power Equation 5 which becomes P. E R1 (6)where P1 is the power absorbed by the load or body 6 at any resonanceload voltage E and R1 is the equivalent parallel resistance of the givenload and is constant for that given load.

, All measurements contemplated by my invention are made in practice bymeans oi a hot wire law measuring device.

In the galvanometer II, the deflection 6 repvoltage caused by theinsertion of a load to the raonanco load voltage. I! we adjust thesensitivity v of the galvanometer after introduction of the load so thatthe sensitivity is proportional to the reciprocal of the equivalentparallel resistance oi the load,-as indicated by the followingequation:

l I a K I (8) where K is a factor of proportionality, then bysubstituting for a in Equation 7, the value of :7 determined by Equation8, we obtain:

6 K'- KI'F: (9)

The voltage E applied to the galvanometer is proportional to the squareof the resonance voltage E of .the circuit, or

I hieg 10 where K4 is a factor of proportionality including K, K and afactor E; and E.

It can be seen by comparingEquation 10 with Equation! that thedeflection of the galvanometer is directly proportional to P1, the powerabsorbed by the load. All that remains, therefore, is to calibrate thegalvanometer in terms oi power units, i. e., watts, and to calibrate thescale ll of the resistance It in such manner that upon adjustment itwill cause the sensitivity values of the resistance ll, forany givensetting is no part of this invention, but it sufiices to state that theequation for shunting galvanometers to alter the sensitivity thereofmaybe used for calculation; that is, for any desired value of R, J theresistance of the shunt could be determined from the following equation:

resented by the pointer 13 thereoils proportional to the voltagemeasured and the sensitivity of the galvanometer or Y where R4 is theresistance 01 the galvanometer, R. is the resistance of the shunt and ais the sensitivity factor oi the galvanometer.

Since R1 for different loads is a direct function 0! e1. (assumingconstant no load resonance voltage and no load circuit conditions) theproper sensitivity control setting of the resistance II for a particularload can be determined by adjustment of the voltage en to the same valueunder any no load circuit condition and then noting the readingcorresponding to the resonance load voltage oi as indicated by thegalvanometer after the load has been introduced and the secondary outputcircuit B retuned.

According to Equation 3, R1 is always proportional to the ratio ofproportionality between and, since is a constant. the relation of thesettings of the resistance R- to the resonance casts v tion of the loadrepresented by the equivalent load voltages may be properly determinedfrom v a measurement of the variation in resonance voltage from any noload value en to any load value a; for a known load. Advantage is takenR1 The procedure in calibrating the resistance It is as follows:

The equivalent parallel resistance R0, repre senting the circuitresistance without a load or object between the electrodes, is'flrstdetermined in terms of the resonance voltage 60. This is done bytuning the secondary output circuit B to resonance by means of variablecondenser l. The galvanometer will indicate a maximum voltage, which bysuitable input control means, for instance, means varying the anodevoltage of the oscillating tubes of the generator, is adjusted'so thatthe pointer I3 is deflected to the full scale reading which, forillustration, we will assume to be 100. 'This reading of the metercorresponds to, or is indicative of, en. The resistance ll beingmaximum, the scale I1 is marked zero A calibrating load is then placedbetween the electrodes and the circuit is again tuned to resonance bythe condenser l. The actual power absorbed by the calibrating loadmay bedetermined according to any known method, as, for example, by acalorimeter. For purposes of illustration it may be assumed that thecalibrating load absorbs 60 watts ofpower, and that the galvanometerpointer indicates the value of 25' for the resonance load voltage e1.The change in the reading of the galvanometer from the value of 100 tothe value of 25 signifies the value of eo-ei (which represents thesensitivity of the gaivanometer) tobeproportional to r in being theequivalent we resistance of the load.

The scale I1. is now marked opposite the po-' sition .of the pointer IIto indicate the proper parallel resistance R1. The galvanometer l2-willregister the correct value of the power absorbed 1 by the load for anyvalue of high frequency voltage applied to the-circuit B by adjustmentof the high frequency source, for instance, by adjustment oi the anodevoltage of the oscillating tubes of the generator A. name any amount ofpower can be supplied to theload by such adjustment of the highfrequency generator A, Once the proper setting of the resistance ll hasbeen determined from the change in the resonance voltage effected by theintroduction of the load, the galvanometer I: will correctly indicatethe power absorption regardless or the value of the power input. Havingdetermined onevalue of the resistance It which converts the galvanometerfrom a voltage indicating instrument into a correct reading watt meterand marked the location of the wiper on the dial I I, the dial can nowbe calibrated into proper divisions throughout its range by computationand without the necessity of utilizing other known loads. It is to beunderstood that the calibrating procedure lust describedis that which.is used in the factory to calibrate the apparatus.

The instrument having been calibrated, the procedure for the measurementof the power absorbed by an unknown load or object placed in the circuitis very simple. The circuit B is tuned to resonance by 'cqndenser 4before the object or loadis placed in the circuit, and the outputcontrol of generator A so adjusted that the Pointer ll of thegalvanometer will indicate a certain reading, for instance, a. fullscale reading. The voltage applied by the circuit is changed by thisadiustment while the frequency is maintained constant. The object isplaced between theelectrode's, and the circuit B again tunedto'resonance. 1 The value. registered by the galvanometer is noted, andthe wiper I! of the resistance is then adjusted until the pointer Itarrives at a mark on the dial I'I corresponding to the value indicatedby the galvanometer. adjustment of the resistance ll changes thesensitivity of the galvanometer proportionally to the equivalentparallel loss conductance register the true value of the power absorbedadjustment has been made such as to cause the ratio position of thewiper II. The scale II for ready voltage of the circuit 8 caused by heintroduc- 7 by the object. Any desired value of power may beadministered, or supplied, to the object by so varying the outputcontrol generator as to vary the value of the high frequency voltageapplied to the circuit 13. The correct values of 1 power by the objectwill be indicated, no matter what amount of power is supplied to theoutput circuit B since, the circuit remains resonant fortthe entire run.The resistance ll isset once for any run; that is, for any particularloador object, and is not thereafter altered duringthat run,'even thoughthe power input, and consequently the power absorption, is varied duringthe rim. Thus any desired amount of power may be administered, orsupplied. to the load or object and the value of the power absorbed bythe load or object will be indicated in brated'meter II.

In Figure 2 of the drawing there is shown another form ofinstmmentality. An electronic I tube E is bridged across the conductorsi and 2 of the output circuit and is convicted with Thepower units bythe call- I once.

themovingcoilllofaeterl 'oithe dynamometer type. The excitation coil 2|of the galvanometer is in the circuit 12 which includes' the variableresistance 23 so that the sensitivity of said galvanometer can be variedby adjusting the re istance and thereby the exciting current.

circuit, the combination of a square law measurlng device connectedacross the oscillatory circuit, said device including a meter, theindicating element of which is adapted tobe deflected proportionally tothe square of the voltage oi the oscillatory. circuit, and a variableresistance in parallel with the meter of said device {or varying thesensitivity of the meter, said resistance being calibrated foradjustment in accordance with the change in the voltage of theoscillatory circuit as indicated by the meter upon the introduction ofthe load and when so adjusted to proportion the sensitivity of the meterto the equivalent load resistance which is added to the circuit by theload. said meter having a dial calibrated to indicate, upon saidadjustment of the resistance, the true value of the power absorbed bythe load. 7

2. An apparatus for measuring the power absorbed by a load from anoscillatory circuit, the combination of a hot wire thermocouple and aaalvanometer for measuring the voltage across the circuit, saidgalvanometer indicating the voltage component of the power absorbed bythe load, and a calibrated resistance in parallel with said galvanometerfor changing the sensitivity thereof in definite relation to saidvoltage component oi the power absorbed so that the corresponding valueof the power absorbed by the load portional to the equivalentconductance added to the circuit by the load.

4. The method of measuring the power absorbed by a load from a highfrequency output circuit energizing the load, including the steps ofimpressing upon an electrical measuring instrument a voltageproportional to the square of the no load resonance voltage of saidcircuit, inserting the load into the circuit and thereby adding anequivalent conductance into the circuit which affects a change in thevoltage impressed upon the instnnnent, and then adjusting thesensitivity of said instrument to a value proportional to saidequivalent conductance inserted into the circuit by the load wherebysaid instrument then the power abindicates a value proportional to 5. Ina device for measuring the power absorbed by a load frolna highfrequency supply circuit, said device comprising a meter connected tosaid circuit and calibrated to indicate the change in the voltage of thecircuit caused by the addition of the load to the circuit, andadjustm'ent means for determining the sensitivity or said meter, saidadjustment means being cali- 'brated to adjust the sensitivity of themeter in accordance with the indicated change in voltage to cause themeter to indicate the power absorbed by the load.

6. In a device for measuring the power absorbed by a load from theoutput circuit of a high frequency variable power source, said devicecomprising means for deriving from said circuit is indicated by thegalvanometer, said relation being defined by the formula 1 I P1=E Ecorresponds to the sensitivity of the galvanometer following theadjustment of the calibrated resistance, R1 corresponds to theequivalent parallel resistance inserted into said circuit by said load,and P1 represents the power absorbed by the load, as indicated by thegalvanometer following the adjustment of the calibrated resist- 3. In anapparatus for determining the power absorbed by a load from anoscillatory circuit. the combination of an electrical device having asquare law characteristic, a variable resistance in parallel with saiddevice, said resistance detel-mining the sensitivity of said electricaldevice, and resistance adjustment means including a scale calibrated toadjust theresistance to a value such that the sensitivity of the deviceis proan electrical quantity proportional to the voltage across thecircuit, clectro-responsive measuring means for indicating the value ofS8.ld quantity, and means for thereafter ad ustmg the sensitivity ofsaid measuring means according to a previously calibrated factor ofproportionality between the indicated value of the quantity and theknown power absorption of a calibrating load causing said quantity to beof equal value, whereby said measuring means then indicates the valueof" the power absorbed by the load for any value of power supplied bysaid source.

'1. In a device for measuring the fractional amount of the total outputof power from a high frequency variable power source that is absorbed bythe load in the output circuit of said source, said device comprising avariable impedance for tuning the circuit to resonance under no load andload conditions, nlcans foroeriving from said circuit unidirectionalelectrical enel-glzat ons proportional to the resonance voltages of thecircuit under no load and load conditions, electroresponsive measuringmeans responsive to said electrical energizations and indicating thevalues of said energizations under no load and load conditlons, andmeans for varying the sensitivity of said measuring means, following theintroduction of the load and the retuning of the circuit, in inverseproportion to an equivalent circuit rmistance causing equal change insaid energize.- tions whereby said 'electro-responsive measuring meansthereafter indicates the value of the power absorbed by the load, andvariations thereof as the power supplied by the source is Varied.

8. In a device for measuring the power absorbed by a load in the tunableoutput circuit of a high frequency variable power source, said devicecomprising means for deriving from said circuit an electrical quantityproportional to the square of the resonance voltage of the circuit underload, electro-responsive-measuring means for indicating the value 01'said'quantity. and

means for adjusting the sensitivity of the measuring means andcalibrated to proportion the sensitivity of said measuring means to anequivalent circuit resistance causing said quantity to be of thesamevalueas that indicated by the measuring means for the particular load,whereby said measuring means indicates the value of the power absorbedby the load for any value of powersup plied by said source. 1

9. In a device for measuring the power absorbed by a load in the tunableoutput circuit of a high frequency variable power source, said devicecomprising means for deriving from said I circuit no load and loadvoltages proportional, re-

spectively, to the resonance voltage of the circuit under no load andloadconditions, electro-responing said variation in the magnitude of thederived load voltage, indicates the value of the power abeases acrossthe circuit, electroeresponsive measuring means for indicating a valuehaving a mathematical relation to said quantity, and means forthereafter adjusting the sensitivity of said measuring means accordingto a previously calibrated factor of proportionality between theindicated value-and the known power absorption of-a calibration loadcausing an identical indication,

whereby said measuring means then indicates the a value of the powerabsorbed by the load for any value of power supplied by said source.

13. In,a device for measuringthe power absorbed by a load in the tunableoutput circuit of a high frequency variable power source, said devicecomprising means for deriving from said circuit an electrical quantityproportional to the square of the resonance voltage of the circuitsorbed by the load for any value of power supplied by said source.

10. In a device for measuring the power absorbed by a load in the outputcircuit of a high frequency variable power source, electro-responsivemeasuring means including a meter for measuring the change in voltageacross the output circuit produced by the introduction of the load toprovide a calibration number indicative of the equivalent conductance ofthe load. and means for then presetting the sensitivity of the measuringmeans according to saidcalibration number,

, said means being calibrated to indicate, when its sensitivity issopreset, the value of the power absorbed by the load for any amount ofpower input.

11. In a device for measuring the power absorbed by a load from a highfrequency supply circuit, said device comprising a meter connected tosaid circuit and calibrated to indicate a value corresponding to thechange in the voltage of the circuit caused by the addition or the loadto the circuit, and adjustment means for altering the sensitivity ofsaid meter, said adjustment means being calibrated to adjust thesensitivity of the meter in accordance with the indicated value to causethe meter to indicate the power absorbed by the load.

12. In a device for measuring the power ab,-

, sorbed by a load from the output circuit of a high frequency variablepower source, said device comprising means for deriving from saidcircuit an electricai quantity proportional to the voltage under load,electro-responsive measuring means for indicating a value proportionalto said quantity, and means for adjusting the sensitivity of themeasuring means and calibrated to proportion the sensitivity 0! saidmeasuring means to the value of an equivalent circuit resistance causingsaid quantity to be of the same value as that indicated by the measuringmeans for the particular load, whereby said measuring means indicatesthe value of the power absorbed by the .load for any value of powersupplied by said source.

14. In a device for measuring the power absorbed by a load in the outputcircuit of a high frequency variable power source, electro-responsivemeans including a meter for indicatinga value corresponding to thechange in voltage' across the output circuit produced by theintroduction of the load to provide a calibration number indicative ofthe equivalent conductance of to indicate, when its sensitivity is soadjusted, the value of power absorbed by the load for any amount ofpower input.

15. The method of measuring the power absorbed by a load in a highfrequency output circuit energizing the load, including the steps oftaking readings on an electrical instrument indicative of the resonantvoltages of the circuit before and after inserting the load, adjustingthe sensitivity of an electrical instrument indicating a valueproportional to the square of the voltage across the circuit inaccordance with an index determined from said readings to render thesensitivity of said instrument proportional to the equivalent lossconductance inserted into the high frequency output circuit by saidload, said instrument thereby indicating a quantity proportional to thepower absorbed by said load.

EUGEN LUI'I'ELMIANN.

